1. a) Field of the Invention
The present invention relates to an objective lens system for microscopes, and more specifically to an objective lens system for microscopes which is composed of a small number of lens elements and has favorable performance.
2. b) Description of the Prior Art
It is generally difficult to favorably correct astigmatism and curvature of field in an objective lens system with a small number of lens elements. Further, when it is necessary to favorably correct also chromatic aberration in the objective lens system for microscopes, refractive indices and dispersing powers of glass materials of the lens elements must be taken into consideration, thereby making the correction of the aberrations more difficult.
When an attempt is made to design an objective lens system for microscopes which satisfies various demands for working distance, magnification, etc. while solving the problems described above, it is necessary to compose the objective lens system of interest of a very large number of lens elements.
In order to solve the problems described above and design objective lens systems for microscopes having favorably corrected aberrations, graded refractive lenses are used. As conventional examples of objective lens systems for microscopes which adopt graded refractive index lenses, there are known the lens systems disclosed by Japanese Patents Kokai Publication No. Sho 62-31816, No. Sho 62-31817 and No Sho 62-34117.
All of these conventional objective lens systems for microscopes use radial type graded refractive index lenses which have refractive index distributions in the radial direction (radial GRIN lenses).
Further, known as lens systems using axial type graded refractive index lenses which have refractive index distributions in the direction along the optical axis are the lens systems proposed by Japanese Patents Kokai Publication No. Sho 61-176905, No. Sho 61-113017 and No. Sho 62-50808. However, these lens systems are not objective lens systems for microscopes, but photographic lens systems or pickup lens systems for optical disks. Lens systems to be used for different purposes of application have different design specifications and adopt GRIN lenses which are different in their outside diameters, thickness, refractive index gradients (absolute values of the refractive index distribution coefficients of the GRIN lenses) and so on. Most of the axial GRIN lenses used in photographic lens systems for cameras, for example, have larger outside diameters and are thicker than those adopted for objective lens systems for microscopes. Further, the refractive index distribution gradients of the GRIN lenses adopted for objective lens systems for microscopes have larger values than those of the photographic lens systems for cameras.